# Suggest a timer chip

I need to take voltage measurements separated by long delays (couple of hours) and beep a buzzer when voltage level is met.

Power consumption is important.

What i want is a simple timer chip like RTC chips, but dumb, without serial interface, memory, etc. What's nice about these serial RTC chips is power consumption in nA range though.

I'm aware, i could use 555, but the lowest power consumption i've found is about 500uA.

Can anyone recommend a simple timer chip with low power consumption?

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Yes, 555 is pretty bad. Even a CMOS device like the TLC555 consumes up to 400$\mu$A. I blame the resistor divider, the other parts can easily be made in the 1-10$\mu$A range.

If I understand your problem correctly you want to monitor a varying voltage and get a signal when it reaches a certain level, and that at low power; I presume because it has to run a long time on a battery.

You don't want a 555, less a microcontroller. You just want a low power comparator. The LPV521 is a Nanopower opamp, requiring 400nA maximum at 5V. No need to switch it on and off. Just apply the voltage to be monitored and the reference voltage to the inputs, and switch a MOSFET which in turn controls the buzzer. Apply positive feedback for the opamp to get a hysteresis to avoid oscillating of the output when the input voltage is around the threshold.

The circuit should consume less than 1$\mu$A, so that it can run for several years on a CR2032 button cell.

edit
Note that to achieve this extremely low power the opamp has a very low bandwidth of 6.2kHz. Here you signal is DC, but in other applications it may matter.

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thanks, this is exactly what i need, i wanted to avoid microcontrollers for application this simple. any ideas on low power buzzer circuit? or ~5mA for general purpose buzzer makes it irrelevant? – miceuz May 29 '12 at 9:07
The buzzer's power is probably not relevant since it will be off all the time: use the MOSFET to switch its power supply, and all you have is the FET's leakage current. (Thanks for the accept. I figured this would have a really simple solution if you take a few steps back to look at it from a distance:-)) – stevenvh May 29 '12 at 9:11
But this doesn't do any timing. If this actually solves the OP's problem, then what was all the stuff about wanting to sample the signal every couple of hours? The comparator method essentially monitors the signal continuously. – Olin Lathrop May 29 '12 at 12:37
@Olin - Yes, I know, but I figured OP was so preoccupied with things like sleep modes to save power that he forgot to take the problem "one level up" (like you often suggest in your answers too). I understand that you're frustrated to have given an answer which doesn't address OP's real needs. (But you got appreciation for it!) – stevenvh May 29 '12 at 14:16
@Olin - In my second paragraph I rephrased OP's question as I understood it. Seemed to have been the right interpretation. He wanted to monitor and get a signal, that's it. The timer is misleading. He talks about RTCs as too, and you know they're misleading as well. (I had to rephrase my comment. I used the idiom red herring, but I don't want to suggest that OP misleaded deliberately. My apologies (not a native English speaker)) – stevenvh May 29 '12 at 14:55

Rather than a stand-alone timer chip, I'd suggest using an extreme low power microcontroller such as the PIC18F24J11. It has a hardware RTC and consumes just 830 nA in sleep mode with the RTC running. It has a 10-bit, 10-channel ADC so you can do your voltage measurements with it also.

Available in a DIP package for prototyping for $3.18, and under$2 in a SMT package in production quantities if this is for a product.

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 Similar but I would suggest a lower cost PIC12F1822 (<\$1 in qty). Going to sleep would get to under 100ua I suspect awakened by a timer to read a voltage. – kenny May 28 '12 at 21:38 @kenny, I assume you meant 100 nA, not uA. With a processor without a hardware RTC like the PIC12F1822, you would still need to keep Timer 1 running, which would consume 650 nA, about the same as my selection. – tcrosley May 28 '12 at 21:59 True. Except the price. – kenny May 28 '12 at 22:32

I agree with what tcrosley said, except that you don't need a real time clock at all. You apparently only want to measure a delay and don't need to know the date and time. A real time clock will be more complicated than just a timer for this task.

Not needing a real time clock also allows for a simpler microcontroller. Any of the "XLP" PICs from Microchip with a A/D can do this. If you need accurate timing, then you put a 32768 Hz crystal on the timer 1 oscillator pins. This is the same kind of crystal used in wrist watches, and can be driven with very little power. Without doing anything special, this can wake the processor every 2 seconds, and the rest is firmware. The processor will only be running a few microseconds every 2 seconds, so the average power consumption will be quite low.

Some of the newer PICs have very low power RC oscillators built in too. That might be all you need if a few percent accuracy is good enough. In either case, this should be doable for around 1 µA or less.

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